The Twins represents the only crossing point over the Green Fork for hundreds of miles in either direction. It is a major barrier to travelers and merchants traveling from the North to the western Riverlands. It lies directly athwart the main route from Winterfell to Riverrun. Moving from one castle to the other whilst avoiding the Twins requires a lengthy detour hundreds of miles to the south or hazardously traversing the bogs and swamps of the Neck to the north.[1]

The Freys' rise began three generations before the events of the series with the building of a stone bridge over the Green Fork. They grew wealthy by charging passing travelers for the use of the Crossing.[2][3]

History

Robb Stark needs to cross the river at the Twins to face Tywin Lannister's forces in battle, but his mother warns him that Lord Walder Frey is proud, prickly and unreliable. She negotiates on Robb's behalf and wins an alliance: in return for allowing Robb's forces to cross and for the allegiance of House Frey, Robb and Arya must marry some of House Frey's offspring. Robb must also take Olyvar Frey as his squire, a knighthood being expected in due course. Robb accepts the deal. His army crosses the river and splits into two forces, a diversionary force to face Tywin and a larger force to ride hard and attack Jaime Lannister's forces north of Riverrun.[4]

The Twins are referenced in conversation between Robb Stark and his eventual bride Talisa Maegyr on the topic of Robb's promised betrothal to any Frey girl of his choosing - upon asked why, Robb declares that The Twins is "a very important bridge".[5]

After the funeral of his grandfather, Robb Stark comes to conclusion that drastic action must be taken to force the Lannisters to surrender by taking Casterly Rock. This plan is hindered by the murder of two Lannister children, Martyn and Willem who were Robb's prisoners by his bannerman Rickard Karstark - causing Robb to take Rickard's head as justice for defying his orders.
This action left Robb without the support of House Karstark and a large portion of his army, so he sought to redeem his favour with House Frey and Walder Frey by marrying his uncle to a Frey girl. As Robb had defied the oath he had took to marry a Frey girl himself, he was on bad terms with the Twins, but all seemed to be forgiven.
The event that occurred became known as the Red Wedding, and revealed the traitorous intentions of House Bolton and Roose Bolton, and the wrath of Lord Walder Frey as he ordered an ambush attack after the bedding ceremony which resulted in the demise of Robb Stark, his wife Talisa Stark and his mother Catelyn Stark, the imprisonment of Edmure Tully and the massacre of some of Robb's bannermen and a large amount of Northern soldiers were killed in the camps outside the Twins.[6]

In the books

In the A Song of Ice and Fire novels, House Frey was a minor house of the Riverlands until six hundred years ago. The Frey's Crossing was initially defended by wooden castles at either end. Using the wealth generated by their toll they replaced the wooden castles with strong stone ones, complete with portcullises, moats and barbicans. The bridge itself is covered and wide enough for two wagons to travel abreast. The Crossing has its own tower, named the Water Tower. House Frey's rise gained them vassal houses of their own: Houses Charlton, Erenford and Haigh.